1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling AC servomotors for use as a driving source of an industrial machine such as a machine tool controlled by a numerical control (NC) device, and a robot.
2. Description of the Related Art
In the case where a large-sized workpiece is driven by a single servomotor as with a C-axis of a crankshaft grinding machine, the workpiece is occasionally twisted to cause torsion due to machining disturbance, and such torsion of the workpiece adversely affects a machining accuracy.
Conventionally, to cope with the torsion of a workpiece, two servomotors are arranged at opposite sides of the workpiece and are driven in synchronism with each other, to reduce the torsion of the workpiece.
As a conventional driving technique, synchronous control is known in which one driving system is controlled by two servomotors coupled to each other. FIG. 11 is a schematic block diagram illustrating conventional synchronous control for two servomotors. In FIG. 11, a main axis and a sub-axis coupled to an unshown workpiece are associated with a main servomotor 25 and a sub-servomotor 35, respectively, the main servomotor 25 being controlled by a main servo circuit 2 while the sub-servomotor 35 being controlled by a sub-servo circuit 3. The servo circuit 2, 3 comprises a position controller 21, 31, a velocity controller 22, 32 and a current controller 23, 33 respectively, and is supplied with an identical position command from a numerical controller.
In the control shown in FIG. 11, to adjust a synchronization error between the main servomotor 25 and the sub-servomotor 35, position feedback values are obtained from encoders 26 and 36 associated with the servomotors 25 and 35, respectively, an adjusting value is derived using the position feedback values, and the adjusting value is added to the position command for the sub-servo circuit 3.
In some cases, high-accuracy control is required for driving servomotors. As such high-accuracy control for controlling servomotors which, for example, perform operation repeated at a predetermined period as in the case of driving the C axis of a crankshaft grinding machine, repetitive control for correcting a predetermined periodic component, which is a sort of learning control, is known.
FIG. 12 illustrates a configuration in which repetitive control is applied to servo circuits, wherein repetitive controllers 27 and 37 are provided in the main servo circuit 2 and the sub-servo circuit 3, respectively, to perform repetitive control for each of the servomotors. Each repetitive controller is input with a position deviation between a position command and a position feedback value and returns its output to the position deviation, to correct the periodic component of the position deviation.
Conventionally, in the case of controlling one driving system by a plurality of servomotors including one main motor (main servomotor) and at least one sub-motor (subordinate servomotor), the position command is adjusted at the numerical control device side for a plurality of servo circuits for controlling the respective servomotors, as shown in FIG. 11, to thereby prevent synchronization error between the main motor and the sub-motor. With the conventional control method, however, since the position command is adjusted at the numerical control device side, the response is poor and it is difficult to perform synchronous control with high accuracy.
In the case of the repetitive control, a repetitive controller is additionally provided for each of the position controllers associated with the main axis and the sub-axis, respectively. Consequently, a lag-module memory, which is an element constituting the repetitive controller, is increased in size, exerting an influence upon the hardware mounting area or the cost, and if the mounting area or the cost is reduced, then the control accuracy lowers. Also, if the same processor is used for the processing of both the main axis and the sub-axis, the processing time increases, imposing restrictions on hardware or cost because of the need for a high-speed processor.
Further, since the position control of the servomotors is performed using a plurality of repetitive controllers, interference between axes occurs in high-rigidity machines, giving rise to a problem that convergence of the position deviation of a workpiece lowers.
Also, in the case where the control is applied to the C axis of a crankshaft grinding machine, the motor coupling is often released to allow attachment and detachment of a workpiece, and in such cases the main motor and the sub-motor need to be operated independently. With the conventional control method, it is difficult to perform switchover between coupled drive mode and independent drive mode.